DE2630819A1 - PROCESS AND LATHE FOR ZONE FIRING OF CERAMIC PIPE MATERIAL - Google Patents

Description

Patent attorneys Dipl.-Ing. H. Weickmann, Dipl.-Phys. Dr. K. Fincke

Dipl.-Ing. F. A. W'kickmann, Dipl.-Chem. B. Huber

8 MUNICH 86, POST BOX 860 820 SbT

MÖHLSTRASSE 22, CALL NUMBER 98 3921/22

CHLORIDE SILENT POWER LIMITED, 52 Grosvenor Gardens, London, SW1W OAU,

England

Process and rotary kiln for zone firing of ceramic pipe material

The invention relates to a method for zone firing ceramic pipe material, which in its longitudinal direction is passed through a rotating tubular furnace. The invention also relates to a rotary kiln for implementation this procedure.

When making long pipes from ceramic material to be fired, for example from α-aluminum oxide, the unfired pieces are usually placed in closed saggars hung that are put in an oven so that

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the tubes are heated evenly along their length. In this way, pipes can be made with good straightness and circular shape for .commercial use. The invention is concerned with the manufacture of pipes under Use of zone firing or zone sintering. Zone burning has been particularly popular in recent years for the Manufacture of tubes from ß-alumina for use as a solid electrolyte in electrochemical cells such as e.g. Sodium-sulfur cells developed.

GB patents 1,297,373 and 1,375,167 and British patent application 11 835/73 describe zone firing processes for producing tubes from β-alumina ceramic in which a piece to be fired is formed from either finely divided crystalline β-alumina or finely divided powders that form the ceramic material when fired. The unfired piece is passed through an electric induction furnace which is tubular with open ends. The piece can be formed, for example, by isostatic compaction of powder or by extrusion of a paste or by electrophoretic deposition. The furnace has a relatively short zone with the predetermined firing temperature (typically about 160 ° -1900 ° C.) and the tube is passed through the furnace at such a rate that the temperature of each tube section is rapidly increased to the firing temperature when it is Part through the _: Burning zone is passed, after which a rapid cooling takes place. This means that every point on the pipe is heated and cooled. A very short burn and heat-up time can be used while maintaining precise temperature control. This enables the production of pipes with fine-grained material and high density. As in the one already

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said British patent application, the furnace may contain a tube made of refractory material, for example di- alumina, through which the tube to be burned is passed. The furnace tube can be rotated continuously to ensure that temperature changes in the hot zone are kept to a minimum, and particularly to ensure that the fired tubes of β-ceramic material are straight and round. Any tendency for the pipe to deform under its own weight is eliminated by this rotation.

In practice, the length of such a furnace is limited since tubular furnaces of the required length are manufactured Cause trouble. Usually a total furnace length of 750 mm with a firing zone of approx. 250 mm length be used. Such a furnace can be used to manufacture tubes from β-alumina with a length of the order of magnitude of 250 mm can be used. If the unfired pieces are much longer, there is poor straightness and round shape, and with a furnace of approx. 750 mm length it is not possible - tubes made of ß-aluminum oxide of, for example 500 mm in length with satisfactory straightness and round shape. It should be noted that straightforwardness and round shape are particularly important for tubes made of ß-aluminum oxide, which are used in sodium-sulfur cells. The ceramic ß-aluminum oxide forms a solid electrolyte, the liquid sodium, which forms the anode, from the cathodic reactant, which consists of liquid sulfur and sodium polysulfides. With such a It is necessary to have the electrolyte tube made of ß-aluminum oxide opposite the cell housing or another cell To seal the construction so that it contains the two liquid electrode substances and every possibility of

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mutual contact is eliminated. The cell operates at a temperature above 250 C, typically of the order of magnitude of 35O ° C. A sealing of the ceramic material on the metal housing or other construction in the case of a cell operating within a wide temperature range, problems arise, and hence accurate dimensional accuracy and shape of the electrolyte tube are very desirable

The reason why long pipes could not be burned so far is that the pipes are connected to a conveyor system into the oven. This was, for example, a band that causes the pipe to move longitudinally. When the unfired piece enters the rotating kiln, it will only start rotating when it is in the kiln pipe is completely absorbed. However, if it is very long, its front end is already inserted into the burning zone before the rotation is started, causing deformation of the pipe. This problem cannot be avoided by that the furnace pipe is made longer as refractory pipes of suitable length are not commercially available.

Of course, zone firing processes are not limited to the production of tubes made of ß-aluminum oxide. They can equally be applied to the manufacture of pipes from other combustible ceramic material, for example cA -alumina pipes such as are used in the ovens described in the above-mentioned British patent application. They can also be used for tubes or rods made of titanium dioxide or magnesia.

The object of the invention is the production of ceramic tubes by the zone firing process with a length

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to allow, which is much larger than the previously possible length.

To solve this problem, a method of the type mentioned is designed according to the invention in such a way that the unfired pipe material before introduction into the furnace in a longitudinal and a rotary movement with the required Speed is offset.

Preferably, after passing through the furnace, the pipe material is arranged on a bearing that supports one of the rotational movements in the Oven generated corresponding rotary movement.

Rotating the pipe to be burned before it enters the furnace ensures that the front end of the Rohrs already has the required speed of rotation when it reaches the combustion zone. Similarly, at the exit of the furnace with the specified storage of the fired tube in the sense of a continuation of its rotation it is ensured that the entire object continues its rotation at the required speed even if the front end has already left the oven. For any given furnace length it is thus possible to use tubes made of ceramic ß-aluminum oxide to burn without deformation with a much greater length than was previously possible.

A rotary furnace for carrying out the method according to the invention, with a rotatable tubular furnace part on which Entrance a feed device for ceramic pipe material is provided, is characterized in further training of the inventive concept in that it feed device a simultaneous longitudinal and rotary movement of the pipe material generated and that the rotary movement about the longitudinal axis

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of the pipe material takes place at the speed required in the furnace.

In one embodiment the delivery device comprises a rotating drive tube with the same inner diameter as the tubular furnace part, which is axially aligned with this and is rotated at the same speed, as well a drive device for feeding the ceramic tube in its longitudinal direction into the drive roller. The drive device preferably comprises a tape and roller bearing on which the pipe to be burned rests, this roller bearing is made so that a rotation of the tube about its longitudinal axis is possible.

The roller bearing can be guided on the belt, so that the tube resting on the rollers forwards through the belt is moved. According to another arrangement, the rollers can be rotatably supported in fixed bearings along the belt. In this case, the drive belt with a sliding element to act on the end of the pipe to be burned and be provided to generate the required longitudinal movement. The rollers can have their axes parallel to the axis of the be arranged to burn pipe, so that the pipe in the longitudinal direction is pushed along the rollers. The rollers can also be arranged with their axes at an angle, which only allows a rolling contact and thus causes the necessary screw movement of the pipe surface. In this case the drive belt is superfluous.

In some cases, the drive belt with the roller bearings can be provided so that the pipe to be burned directly is guided into the rotatable tubular furnace part. The arrival

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drive with roller bearings allows easy rotation of the tube so that it is rotated through the tubular furnace part once it is partially received therein. Consequently the invention also includes a rotary kiln for firing ceramic tubes with a rotatable tubular furnace part the one at the entry end of a drive belt for supplying to burning pipes in the longitudinal direction in the rotatable furnace part and roller bearings are provided on which the to burning tube and which allow a rotation of the tube about its longitudinal axis. The roller bearings can be arranged in the manner described on the drive belt or rotatable about fixed axes. These axes of rotation of the roller bearings can be arranged at an angle, so that only a rolling contact on the helically moving surface of the pipe to be burned.

An adjustable fixed element, for example a short pipe, can be used to guide a pipe to be burned into the Oven be provided. With zone firing, it is beneficial to leave only a small distance between the pipe to be burned in the unfired one Form and the tubular furnace part, and an adjustable guide element can ensure the correct entry of the unfired pipe material in the furnace. The setting may then be required, for example when the dimensions of the pipe material to be burned are changed.

In all of the possible embodiments described above, the required rotational speed is the one to be burned Rohrs as it enters the furnace in such a way that its circumferential linear velocity is practically equal to that of the inner surface of the tubular furnace part is. Since the pipe to be burned rotates by a drive on its circumferential surface

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is shifted, the rotational speed of the drive device is always in a fixed ratio to the rotational speed of the tubular furnace part, regardless of the Diameter of the pipe to be burned. It is therefore easily possible to generate the required speeds and adhere to.

Preferably, the furnace further has a rotating outlet pipe at the outlet end, the inner diameter of which is equal to that of the tubular furnace part and which is driven at the same speed. It's lengthways aligned with the tubular furnace part.

In some cases, however, it may be sufficient at the exit of the furnace to provide a bearing that allows a longitudinal and a rotary movement of the fired tube. This can be, for example be a drive belt with freely rotating rollers either on the belt or on fixed bearings. In the latter In the case, the rollers can be provided with axes aligned in such a way that a rolling contact between them and the Allow helically moving surface of the fired tube.

The drive pipe at the inlet and the outlet pipe do not have to be made of refractory material, as the one to be burned Tube is cold before entering the furnace and cools down quickly when leaving the tubular furnace part. Advantageous therefore the drive tube and the outlet tube are made of glass. This enables observation of the in the Objects introduced into and exiting the furnace.

The drive belt advantageously comprises two groups of axially aligned ones Rollers with axes parallel to each other, where

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in each case one group is arranged on one side of a flexible carrier. The rollers are freely rotatable and arranged so that the pipe to be burned can rest on them, with its longitudinal axis parallel to the two groups of rollers and runs between them. The flexible carrier can serve as a bearing for the rollers or a drive element wear, for example a slide that acts on the pipe. The carrier can lie between or under rollers, on which the pipe to be burned rests. The drive belt is advantageously a flexible belt. It is convenient as a running endless belt and arranged at the entrance of the drive tube, from this on an arc of a circle is led downwards. The roller bearings on the belt can be curved with their peripheral surfaces in the axial direction of the rollers, a radius of curvature being provided, which is equal to the radius of the aforementioned circular arc.

In practical use, a series of tubes to be burned is passed through the furnace one after the other, with each tube after leaving the drive belt is pushed through the furnace by the following pipe. Thus there is no longitudinal drive required in the oven. A single tube can be passed through the furnace by providing a suitable slide which is held on the drive device. Such a slide can also be used to push the last one through Rohrs a flow rate to be burned tubes are used.

An embodiment of the invention is described below with reference to the figures. Show it:

1 schematically shows an induction furnace for zone firing of ceramic tubes,

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Fig. 2 is a perspective view of a part

a drive belt for the ge in Fig. 1. showed furnace,

3 shows another embodiment of a drive belt and the roller bearings,

Fig. 4 schematically shows a further embodiment of a drive belt and the roller bearings and

5 shows a further construction of a furnace with an associated feed device.

In Fig. 1, an induction furnace for zone burning of tubes made of ß-aluminum oxide is shown schematically, the one insulated furnace box 10 and a graphite receiving block 11 comprises. An induction coil 14 powered by a high frequency source 15 is fed, surrounds the receiving block 11, as also described in the documents mentioned above is, which is why this is also referred to with regard to the further details of an induction furnace. Through the Receiving block 11 and the furnace box 10, a tubular furnace part 12 is passed through, for example made of recrystallized Consists of alumina. This part of the furnace is running at a constant speed by means of drive rollers 13 rotated as described in the aforementioned British patent application. A speed of rotation from 15 to 60 rpm can be provided.

At the entrance of the furnace, a rotating inlet tube 20 is provided, which over the greater part of its length Has an inner diameter which is equal to that of the tubular furnace part 12. This inlet pipe 20 is axially on the

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tubular furnace part aligned and is rotated in the same direction and at the same speed as this by means of drive rollers 21. The end of the inlet pipe 20 facing away from the furnace is widened outwards at 22 in order to be able to receive pipes that are to be burned. The inlet pipe is advantageously made of glass. The pieces of ß-alumina to be burned, which are produced from ß-alumina (or from a material which forms ß-alumina when heated) by isostatic pressing of finely divided ceramic material, are introduced into the rotated inlet tube 20 by means of a drive belt 24 which is inserted into Further details are shown in FIG. This figure shows an endless drive belt which moves in its longitudinal direction according to the illustrated wedge X and carries two groups of rollers 25 which are fastened to a flexible carrier 26 in a freely rotatable manner. The two groups of rollers are each arranged on one side of the belt, and the rollers on each side are axially aligned with one another. The axes of the two groups of rollers are parallel to each other. The dimensions of the rolls are such that the pipe to be burned rests on the rolls and not on the flexible carrier 26. Thus, the pipe to be burned can rotate freely on this drive belt, although it is moved by it in the longitudinal direction. As can be seen from FIG. 1, the drive belt 24 is led away downwards on a curved path near the widened entry of the inlet pipe 20. The drive belt 24 runs over an arc of a circle with the radius A, and the individual rollers 25 on the belt have peripheral surfaces 29 which are convexly curved in the axial directions of the rollers (Fig. 2), the curvature having the radius B (Fig. 1) has _? This ensures that with the curved course of the drive belt at the end of the carrier system, the relative movement between the individual rollers and the pipe to be burned is a rolling movement.

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It can be seen that the tape carrier system to be burned Pipes in the rotating inlet pipe 20 leads. When the front end of the respective pipe to be burned into the rotating Entrance pipe enters and is carried by this, the pipe takes up the rotary movement, which is caused by the rollers 25 becomes possible on the drive belt. Thus, the pipe to be burned will be rotated at the exact speed when it already enters the tubular furnace part 12.

At the exit end of the furnace is a rotating exit pipe

27 is provided, which is also advantageously made of glass. This tube has the same inner diameter as the tubular one Part of the furnace and is axially aligned with this. It is driven by rollers 28 so that it is moving at the same speed and rotates in the same direction as the tubular furnace part 12. When the burned ß-alumina pipe leaves the tubular furnace part 12, it is stored so that it is possible to continue its rotation. The outlet pipe 27 is long enough so that the rotation of the pipe passed through the furnace continues for the entire time during which every part of this tube dwells within the burn zone.

If, in the embodiment shown in FIGS. 1 and 2, the inlet pipe 20 and the outlet pipe 27 are the same Diameter and the same thickness as the tubular furnace part 12, so it is easily possible to determine the exact rotational speeds adjust by turning the drive rollers 13, 21 and

28 have the same diameter and are driven by a common shaft or by separate drive shafts rotating at the same speed.

Instead of fixing the roller bearings on the drive

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tape, as shown in Fig. 1 and 2, these can also be rotatably mounted on fixed axes, as shown in Fig. 3 is shown. It is a flexible drive belt 30 with a pair (or more pairs) of parallel driven rollers 31 shown on which a pipe 32 to be burned rests. These rollers 31 are driven at a suitable speed so that the required rotational speed for the tube 32 is created as it enters the furnace or in a rotating entry tube in front of the furnace. The band 30 creates a longitudinal movement of the tube 32, and for this purpose an adjustable sliding element 33 is provided, which is shown in FIG one of several openings 34 of the belt 30 can be used can and acts on the rear end of the tube 32.

In the embodiment shown in FIG. 3, the tubes must slide along the rollers 31. As shown schematically in Fig. 4, can rollers 35 to support a tube 36 with their axes be arranged so that a pure rolling contact between the tube 36 and the rollers 35 occurs. Of the The angle of the roller axes then corresponds to the helical movement of the pipe surface.

D. The belt drive and the roller supports have been described above for the special application of guiding a pipe written in an oven. Similar constructions can also be provided at the exit of the furnace. Especially can A rotating outlet pipe may be dispensed with here if a support device with freely rotatable or powered rollers is provided. Rotatable tubes at both ends of the furnace are particularly suitable for long burning pipes. For intermediate lengths, it may be sufficient to provide a rotatable tube only at one end of the furnace.

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Fig. 5 shows a further embodiment of a zone kiln, in which, similar to that shown in FIG. 1, a tubular furnace part 12 made of <X -aluminium oxide on drive rollers 13 is mounted and runs through a receiving block 11 within an insulated furnace box 12. An induction coil 14 surrounds the receiving block 11 and is powered by a high frequency source 15 fed. As with the device shown in Figure 1, the furnace is at an angle of 7.5 ° from the inlet end arranged rising upwards, whereby a convection air flow is generated through the furnace towards the outlet.

In Fig. 5, a plurality of ceramic tubes 40 are one behind the other shown, they are passed through the furnace with spacers 41 so that each tube through the respective subsequent tube is advanced. The tubes and the spacers are arranged one after the other on two horizontally spaced apart parallel drive rollers 42 arranged. These rollers have such a distance from one another that the tube 40 rests on them and its axis is arranged parallel to the roller axes and on the longitudinal axis of the tubular furnace part 12 is aligned. The rollers 42 have the same diameter as the drive rollers 13 and are with the same Speed rotated. They are advantageously arranged in pairs coaxially with the rollers 42 so that common drive axes used v / ground.

, The rollers 42 are made of steel and have an aluminum oxide coating, so that the β-alumina is not damaged. In general, the roles consist of a material which does not damage the ceramic material to be fired or only generates negligible impurities «A sliding block 43 is provided for longitudinal movement. This sliding block is driven by a chain drive and consists of one

thermosetting resin. It is provided with two rubber rings 44, so that it rotates with the rollers 42 and assists the rotation of the tube 40 which would otherwise be on the smooth Surface of the rollers 42 could be interrupted.

The rotational speed is typically for the tubular furnace part 12 25 rpm. The rollers 42 are slightly longer than the pipes to be burned so that each pipe rotates at the required speed while it is on the rollers rests and moves along the rollers. A short glass guide tube 45 is between the rollers 42 and the entrance provided in the tubular furnace part. During operation, the guide tube 45 is held, but it is mounted so that that it can be adjusted to ensure that the unfired tubes fed by the rollers 42 are exactly in the tubular furnace part 12 can be inserted. In practice it is beneficial to have only a very small distance between the green pipes and the tubular furnace part 12, and the adjustment of the guide pipe may be required if the furnace is used for firing pipes of other dimensions.

At the outlet end of the furnace, a glass outlet pipe 46 is arranged on driven rollers 47 which are made of synthetic resin glued asbestos material.

The method described above and the devices for carrying it out can also be used for burning cylindrical or tubular compacted objects of other ceramic materials are used in a duct furnace.

The following table shows comparative measurements to clarify the better rounding and straightness of a tube made according to the invention. ß-alumina.

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In all measurements, the tubes had a diameter of 33 mm and a length of 440 mm.

Furnishing Gemes
sene
number Straight
speed mm (5) End ge
closed
Outside 0 Round shape (6) mm end
open minded
Outside 0 end
open minded
Inside 0 tion
(see below) 5
8th
10
6th 1.8 * 0.6
1.4 * 0.4
1.0 * 0.2
1.0 * 0.2 0.3 * 0.2
0.3 * 0.3
0.3 * 0.1
0.1 * 0.1 center
Outside 0 0.8 * 0.7
0.3 * 0.3
0.2 * 0.1
0.01 * 0.01
(7) 0.7 * 0.5
0.2 * 0.1
0.1 * 0.1 1
2
3
4th 0.4 * 0.2
0.2 * 0.1
0.2 * 0.1
0.1 * 0.1

1. No rolls at the inlet or outlet end of the furnace, unfired tubes fed with open faces facing each other or closed ends.

2. Aluminum roll at the furnace inlet, synthetic resin-bonded asbestos roll at the furnace outlet, unfired pipes supplied with facing open and closed ends.

3. As in 2, but pipes fed with the open end in front by means of ß-aluminum oxide spacers.

-4. Like 3, but instead of aluminum rollers, rollers coated with aluminum oxide.

5. Measured as the maximum deviation from straightness; Pipes are usually evenly bent.

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Diameter; usually at an angle of

6. Measured as the maximum diameter minus the minimum diameter to each other.

7. After diamond grinding.

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Claims (24)

Claims 1. Process for zone firing ceramic pipe material, which is in its longitudinal direction by a rotating tubular furnace is performed, characterized in that the unfired pipe material before the introduction into the furnace in a longitudinal and a rotary movement at the required speed will. 2. The method according to claim 1, characterized in that that the pipe material is arranged on a bearing after passing through the furnace, which one of the rotary movement corresponding rotary movement generated in the furnace. 3.) Rotary furnace for performing the method according to claim * """1 or 2, with a rotatable tubular furnace part, at the inlet of which a feed device for ceramic pipe material is provided, characterized in that the feed device (24) has a simultaneous longitudinal and Rotational movement of the pipe material (32) is generated and that the rotational movement about the longitudinal axis of the pipe material (32) takes place at the speed required in the furnace. 4. Rotary furnace according to claim 3, characterized in that that the feed device comprises a rotatable drive tube (20), the inner diameter of which corresponds to that of the tubular furnace part (12) which is axially aligned with the tubular furnace part (12) and is rotated at its rotational speed, and that a drive device (24) for guiding the to be burned Rchriiiaterials (32) Iu longitudinal direction in the drive rc.hr- (20) is provided. 30S884 / 0S8T 5. Rotary kiln according to claim 4, characterized in that that the drive device comprises a drive belt (24) and roller bearings (25) on which the to be burned Pipe material (32) rests, and that the roller bearings (25) a rotation of the pipe material to be burned (32) allow about its longitudinal axis. 6. Rotary furnace according to claim 5, characterized in that that the roller bearings (25) are arranged on the drive belt (24), so that on the roller bearings (25) overlying Rohrmatcrial (32) is conveyed by the drive belt (24). 7. Rotary furnace according to claim 5, characterized in that that the roller bearings (25) are arranged in fixed holders along the drive belt (24). 8. Rotary kiln according to claim 7, characterized in that that the drive belt (30) with a sliding element (33) to act on the end of each burning pipe material (32) and is provided for generating the required longitudinal movement. 9. Rotary kiln according to one of claims 6-8, characterized in that the roller bearings (25) are arranged with their axes parallel to the longitudinal axis of the pipe material (32) to be burned. 10. Rotary kiln according to claim 7 or 8, characterized in that the roller bearings (35) under one Angle to the longitudinal axis of the pipe material to be burned (32) are arranged, so that a pure rolling contact between the pipe material (32) and the roller bearings (35) for the required helical movement of the . Pipe surface is generated. - 20 603884/0887 11. Rotary kiln according to claim 3, characterized in that that the feeding device comprises rollers (25) arranged on fixed supports, that the rollers (25) in zv / ei lines are arranged parallel to the longitudinal axis of the tubular furnace part (12) on both sides of this axis, that the axes of the rollers (25) at an angle of less than 90 ° to the longitudinal axis of the tubular furnace part (12) are arranged and that a device (24) for Longitudinal movement of the pipe material to be burned (32) is provided so that a rolling contact between the pipe material (32) and the rollers (25) leads to a helical movement of the pipe surface. 12. Rotary kiln according to one of claims 3 to 11, characterized by a drive belt (24) provided at its inlet for feeding pipe material (32) to be burned in its longitudinal direction and by roller bearings {25) on which the pipe material (32) rests and which are arranged so that a rotation of the pipe material (32) about its longitudinal axis is possible. 13. Rotary kiln according to claim 12, characterized in that that the roller bearings (25) are attached to the drive belt (24). 14. Rotary furnace according to claim 12, characterized in that that the roller bearings (25) are arranged to be rotatable about fixed axes. 15. Rotary kiln according to claim 14, characterized in that that the axes of rotation of the roller bearings (35) are arranged at an angle to the direction of movement of the pipe material (32) are, so that a pure rolling touch for the - 21 - 609884/0887 helically moved pipe surface takes place. 16. Rotary kiln according to claim 12, characterized by a drive for rotating the roller bearings and thus the pipe material to be burned (32) about its longitudinal axis, which has a linear peripheral speed of the Tubing material (32) generated which corresponds to that of the tubular furnace part (12). 17. Rotary kiln according to one of claims 11 to 16, g e code Inet by an adjustably attached guide element (45) for the introduction of a to be burned Pipe material (40) from the feed device (42) into the tubular furnace part (12). 18. Rotary furnace according to one of claims 3 to 17, characterized by a rotatable outlet pipe (27) at the outlet end with an inner diameter equal to that of the tubular furnace part (12) and by a device (28) for moving the outlet pipe (27) at the speed of rotation of the tubular Furnace part (12), the outlet pipe (27) being axially aligned with the tubular furnace part (12). 19. Rotary kiln according to one of claims 3 to 18, characterized in that at the exit of the rotary kiln a device enabling the longitudinal movement and the rotational movement of the fired tubular material (32) is provided. 20. Rotary furnace according to claim 19, characterized in that that the longitudinal and the rotary movement enabling device a drive belt with freely rotatable 609884/088 Is rollers, which are arranged on the belt or freely rotatable in fixed mountings. 21. Rotary kiln according to one of "Claims 5 to 10 or 20, characterized by a drive belt (24) with two groups of axially aligned rollers (25) with parallel axes, each group being arranged on one side of a flexible carrier (26) _s by a freely rotatable fastening of the rollers (25) and by such an arrangement of the rollers (25) that the pipe material (32) to be burned rests on the rollers (25) with its longitudinal axis parallel to the axes of the two roller groups and between them . 22. Rotary kiln according to claim 21, characterized in that that the flexible carrier (26) serves as a storage for the rollers (25). 23. Rotary furnace according to claim 21, characterized in that that the flexible carrier (30) carries a drive element (33) which acts on the pipe material (32) to be burned and is arranged between or below the rollers (25, 31) provided for supporting the pipe material (32). 24. Rotary furnace according to one of claims 5 to 10 or 20 to 23, characterized _? that the drive belt (24) is a flexible endless belt. Blank page